1. Field of the Invention
Present invention relates to a focal length measuring device for measuring a focal length of an optical system.
2. Description of Related Art
For a device for measuring a focal length in an optical system such as a lens, conventionally, a focal length measuring device is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 7-55638 is proposed.
As shown in FIG. 13, the focal length measuring device has a following structure. In the drawing, one point on an optical axis L of a lens TL to be tested is an object point O, and the other point on the optical axis L is an image point I which is formed by the lens TL to be tested. A front focal point of the lens to be tested is indicated by F. A rear focal point of the lens to be tested is indicated by F′. A distance between O and F is indicated by Z. A distance between F′ and I is indicated by Z′. Under such condition, a relationship such asZZ′=f2is effective.
Here, f indicates a focal length of the lens TL to be tested. The above relationship is known as a Newton's formula. As understood from the Newton's formula, it is possible to measure the focal length of the lens TL to be tested by measuring Z and Z′ along the optical axis of the lens TL to be tested accurately.